The creature, officially named Paludirex vincenti, measured more than 16 feet (five meters) long and dominated waterways in southeastern Queensland, according to a press release from the University of Queensland (UQ) published Monday.
It lived between 5.33 and 2.58 million years ago, researcher Jorgo Ristevski, a PhD candidate at UQ's School of Biological Sciences, told PeerJ, the journal which published the research.
Researchers identified the giant crocodile from fossils dug up in the 1980s near a town named Chinchilla.
The species is named after Geoff Vincent, who found a fossilized skull of the prehistoric animal. "Paludirex" means swamp king in Latin and "vincenti" honors Vincent, according to Ristevski,
"The 'swamp king' was one intimidating croc," said Ristevski in the press release. "Its fossilized skull measures around 65 centimeters, so we estimate Paludirex vincenti was at least five meters long."
The largest living crocodile, the Indo-Pacific crocodile -- Crocodylus porosus -- grows to about the same size, he added.
"But Paludirex had a broader, more heavy-set skull so it would've resembled an Indo-Pacific crocodile on steroids," said Ristevski.
The species was one of the top predators in Australia at the time it lived, and would have been able to eat giant prehistoric marsupials, according to the press release.
Two species of crocodile -- Crocodylus porosus and Crocodylus johnstoni -- still live in Australia today, and it is not clear why Paludirex vincenti died out.
"Whether Paludirex vincenti went extinct as a result of competition with species like Crocodylus porosus is hard to say," said Steve Salisbury, senior lecturer in the School of Biological Sciences and Ristevski's supervisor at UQ.
"The alternative is that it went extinct as the climate dried, and the river systems it once inhabited contracted -- we're currently investigating both scenarios."
In September, an enormous saltwater crocodile (Crocodylus porosus) measuring 14.4 feet (4.4 meters) was caught at a remote tourist hotspot in Australia's Northern Territory.
The crocodile, estimated to weigh 771 pounds (350 kilograms), was captured by wildlife rangers at a trap in the Flora River Nature Park, a popular tourist destination southwest of the outback town of Katherine.